Better argument validation for core methods.

test/string/contains.wren

@@ -4,5 +4,3 @@ IO.write("something".contains("meth")) // expect: true
 IO.write("something".contains("some")) // expect: true
 IO.write("something".contains("ing"))  // expect: true
 IO.write("something".contains("math")) // expect: false
-
-// TODO: Passing non-string as argument.

test/string/contains_argument_not_string.wren

@@ -0,0 +1 @@
+"foo".contains(1) // expect runtime error: Argument must be a string.

test/string/subscript.wren

@@ -9,16 +9,3 @@ IO.write("abcd"[-4]) // expect: a
 IO.write("abcd"[-3]) // expect: b
 IO.write("abcd"[-2]) // expect: c
 IO.write("abcd"[-1]) // expect: d
-
-// Handle out of bounds.
-// TODO: Should halt the fiber or raise an error somehow.
-IO.write("abcd"[4])  // expect: null
-IO.write("abcd"[-5]) // expect: null
-
-// Handle wrong argument type.
-// TODO: Should halt the fiber or raise an error somehow.
-IO.write("abcd"[true]) // expect: null
-
-// Handle non-integer index.
-// TODO: Should halt the fiber or raise an error somehow.
-IO.write("abcd"[1.5]) // expect: null

test/string/subscript_not_int.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a[1.5] // expect runtime error: Subscript must be an integer.

test/string/subscript_not_num.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a["2"] // expect runtime error: Subscript must be a number.

test/string/subscript_too_large.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a[4] // expect runtime error: Subscript out of bounds.

test/string/subscript_too_small.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a[-5] // expect runtime error: Subscript out of bounds.